Electronic Structure Of SrTiO₃-based Heterointerface: Theoretical Study

Thermoelectric materials could achieve the transformation between electric energy and thermal energy and all of them have Seebeck effect and Peltier effect. Seebeck effect is a kind of effect relative to the difference of temperature and electricity, means that an electric potential difference would emerge when a tempareture diffierence was exerted on the different side of the materials. Peltier effect is the inverse effect of Seebeck effect, which means that phenomena of absorbing or releasing thermal energy at the different side of thermoelectric materials would happen if electric current of a certain direction passes through these materials. Thermoelectric materials have extensive applications. They can be taken advantage of not only in the field of tempareture measurement and power generation, but also in cooling electronic devices (such as infrared and far infrared detectors, high speed chips and so on) and in the field of medical appliance.Power generation using thermoelectric materials is not only the main energy source of deep space exploration recently, but also could make waste heat from industries into electric energy, which have an extensive foreground. Currently, most of thermoelectric materials have their figure of merit ZT values about 1, corresponding to the conversion efficiency between electric and heat at only about 7%～8%, which is unfortunately not large enough for large-scale usage. Therefore, one aim for thermoelectric material researchers is to enhance ZT value. Recently, two methods have been employed to enhance ZT value, one is to enhance the thermopower or to decrease the thermal conductivity, and the other one is to modify the mirco-structure of materials in order to enhance the thermoelectric figure of merit. In the recent years, two dimensional electron gases (2DEGs) with high density and high mobility were found in many experimental works at the interfaces of oxide heterointerfaces such as TiO2/SrTiO3 and SrTiO3/SrTi1-xNbxO3, corresponding to giant Seebeck coefficients and ZT values, which attracted lots of attention from researchers. With the never-ending changes and improvement of the theories of numerical algorithm and computer technology, first-principle calculations based on the density-functional-theory have already been regarded as an important research method in condensed matter physics, quantum chemistry and materials science. This thesis mainly focus on the microstructure and electron properties of several oxide heterointerface, especially including the investigation on the average potential and band offsets of each superlattice. I not only studied the mechanism and the origin of the 2DEGs at the interfaces and revealed the essence of the experimental result, but also predicted the approach to enhancement the thermoelectric figure of merit.TiO2/SrTiO3 heterojunction:First, many reasonable interface structures have been built and the most stable one have been determined through calculating the binding energies and interfacial energies. Second, many properties of the most stable interface have been studied including the band structure, density of states, charge transfer and found that about 0.3 electrons transferred from TiO2 side to SrTiO3 side, which indicated a good absorption of TiO2 onto SrTiO3 substrate. The average potential and band offsets have also been calculated and the valence band offset and conduction band offset is 0.09eV and 0.19eV, respectively. In additional, a quantum well was found just at the interface due to the discontinuity of the polarization between the two sides of the interface. At last, some quantitive research about the oxygen defect at the interface has been carried out. The results indicated that the oxygen defect could make the interface n-type, which showed metallic states, and the macroscopic static average was increased.SrTiO3/LaAlO3 heterojunction:Two kinds of different interfaces have been built: the AlO2/LaO/TiO2 interface and the AlO2/SrO/TiO2 interface. The geometric structures and electronic properties of each interface have been investigated systematically. Both of the two interfaces were non-neutral, the AlO2/LaO/TiO2 interface was n-type and the AlO2/SrO/TiO2 interface was p-type. The AlO2/LaO/TiO2 electron compensate interface had larger atom displacement at the interface than that of the AlO2/SrO/TiO2 interface.The band structure and DOS of the AlO2/LaO/TiO2 interface indicated that the metallic states was mainly attribute to Ti 3d electrons. At last, polarizations and band-offsets of the mix-style superlattice which contained both the AlO2/LaO/TiO2 interface and the AlO2/SrO/TiO2 interface have been researched. The direction of polarization reversed from one side to another, which could explain the phenomenon of the 2DEG at the n-type interface. The valence band offset and conduction band offset is 0.15eV and 2.45eV, respectively.SrTiO3/SrTi1-xNbxO3/SrTiO3 quantum well (QW) structure:The band structure indicated that the Fermi energy located into the conduction band, therefore the superlattice showed metallic states. The calculation of the average potential confirmed that this superlattice was a typical QW structure. The main position of the QW is at the SrTi1-xNbxO3 layer, and the width and depth of the QW is both increased with the increase of the Nb-doping concentration. Atom displacements of Ti atoms lead to grads of polarizations at the interface. The closer to the interface, the larger of the polarization is. Due to the attraction of the polarization charge, the 2DEG inside the QW diffused into the barrier layers. At last, through calculating the effective mass of the electrons inside the QW, I found that the Seebeck coefficient inside the QW could enhance if the SrTiO3 barrier become thicker, which agreed well with experimental works.